High-precision sampling for Brillouin-zone integration in metals

Abstract:The role that grain boundaries (GB) can play on mechanical properties has been studied extensively for metals and alloys. However, for covalent solids such as boron carbide (B 4 C), the role of GB on the inelastic response to applied stresses is not well established. We consider here the unusual ceramic, boron carbide (B 4 C), which is very hard and lightweight but exhibits brittle impact behavior. We used quantum mechanics (QM) simulations to examine the mechanical response in atomistic structures that model GBs in B 4 C under pure shear and also with biaxial shear deformation that mimics indentation stress conditions. We carried out these studies for two simple GB models including also the effect of adding Fe atoms (possible sintering aid and/or impurity) to the GB. We found that the critical shear stresses of these GB models are much lower than for crystalline and twinned B 4 C. The two GB models lead to different interfacial energies. The higher interfacial energy at the GB only slightly decreases the critical shear stress but dramatically increases the critical failure strain. Doping the GB with Fe decreases the critical shear stress of at the boundary by 14% under pure shear deformation. In all GBs studied here, failure arises from deconstructing the icosahedra within the GB region under shear deformation. We find that Fe dopant interacts with icosahedra at the GB to facilitate this deconstruction of icosahedra. These results provide significant insight for designing polycrystalline B 4 C with improved strength and ductility.

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“…35 While, the Methfessel-Paxton scheme 36 was applied to determine the electron partial occupancies for the Fe-doped GB model.…”

Section: Computational Methodsologymentioning

confidence: 99%

Shear-Induced Brittle Failure along Grain Boundaries in Boron Carbide

Yang

Coleman

LaSalvia

et al. 2018

ACS Appl. Mater. Interfaces

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“…The plane wave cut-off energy was truncated at 400 eV. Methfessel-Paxton smearing, with a sigma value of 0.01 eV, was implemented to improve convergence [53].…”

Section: Methodsologymentioning

confidence: 99%

Application of a parallel genetic algorithm to the global optimization of medium-sized Au–Pd sub-nanometre clusters

2018

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Abstract. To contribute to the discussion of the high activity and reactivity of Au-Pd system, we have adopted the BPGA-DFT approach to study the structural and energetic properties of medium-sized AuPd sub-nanometre clusters with 11-18 atoms. We have examined the structural behaviour and stability as a function of cluster size and composition. The study suggests 2D-3D crossover points for pure Au clusters at 14 and 16 atoms, whereas pure Pd clusters are all found to be 3D. For Au-Pd nanoalloys, the role of cluster size and the influence of doping were found to be extensive and non-monotonic in altering cluster structures. Various stability criteria (e.g. binding energies, second differences in energy, and mixing energies) are used to evaluate the energetics, structures, and tendency of segregation in sub-nanometre Au-Pd clusters. HOMO-LUMO gaps were calculated to give additional information on cluster stability and a systematic homotop search was used to evaluate the energies of the generated global minima of monosubstituted clusters and the preferred doping sites, as well as confirming the validity of the BPGA-DFT approach.

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“…The conjugate-gradient algorithm [15] is used in this computational scheme to relax the atoms. The tetrahedron method with Blöchl corrections [12] as well as a Methfessel-Paxton [16] scheme were applied for both geometry relaxation and total energy calculations. Brillouin-zone (BZ) integrals were approximated using the special k-point sampling.…”

Section: Computation Methodsmentioning

confidence: 99%